This invention relates to acoustic ink printing and, specifically, to an improved acoustic ink printhead with an integrated liquid level control layer and method of manufacture therefor.
In acoustic ink printing, acoustic radiation by an ejector is used to eject individual droplets on demand from a free ink surface. Typically several ejectors are arranged in a linear or two-dimensional array in a printhead. The ejectors eject droplets at a sufficient velocity in a pattern so that the ink droplets are deposited on a nearby recording medium in the shape of an image.
A droplet ejector employing a concave acoustic focusing lenses is described in U.S. Pat. No. 4,751,529, issued on Jan. 14, 1988 to S.A. Elrod et al., and assigned to the present assignee. These acoustic ink ejectors are sensitive to variations of their free ink surface levels. The size and velocity of the ink droplets which are ejected are difficult to control unless the free ink surfaces remain within the effective depth focus of their droplet ejectors. Thus the free ink surface level of such a printer should be closely controlled.
To maintain the free ink surfaces at more or less constant levels, various approaches have been proposed for acoustic ink printers One approach is the use of a closed loop servo system for increasing and decreasing the level of the free-ink surface under the control of an error signal which is produced by comparing the output voltage levels from the upper and lower halves of a split photo-detector. The magnitude and sense of that error signal are correlated with the free ink surface level by the reflection of a laser beam off the free ink surface to symmetrically or asymmetrically illuminate the opposed halves of the photodetector depending upon whether the free ink surface is at a pre-determined level or not. This approach is somewhat costly to implement and requires that provision be made for maintaining the laser and the split photo-detector in precise optical alignment. Moreover, it is not well-suited for use with larger ejector arrays because the surface tension of the ink tends to cause the level of the free surface to vary materially when the free surface spans a large area.
Another approach is described in U.S. Pat. No. 5,028,937, issued on Jul. 2, 1991 to Butrus T. Khuri-Yakub et al., and assigned to the present assignee. In that patent application, an acoustic ink printhead has a pool of liquid ink having a free surface and intimate contact with the inner face of a perforated membrane. The perforations form large diameter apertures which are aligned with respective focused acoustic ejectors. Surface tension causes the ink menisci to extend across each of the apertures at substantially the same level. During an operation an essentially constant biased pressure is applied to the ink to maintain the menisci at a predetermined level.
However, some problems with the membrane perforation technique are difficulties associated with the misalignment of the apertures in the membrane with the acoustic ejectors, warpage of the membrane from an ideal flat surface, and variations in the distances between each aperture and corresponding ejector. Additionally, the edges of the perforations may sometimes be ragged, which can disturb the free surface of the ink so that the uniformity and quality of the ejected droplets are not consistent. Therefore alternate approaches for controlling the ink levels of the free surface for the ejectors are desirable.
The present invention provides for such an alternate approach.